Update masters after the tryserver split.

native_client_sdk/PRESUBMIT.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Top-level presubmit script for isolate.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts for
 details on the presubmit API built into gcl.
 """
 
@@ skipping 22 matching lines @@
 def CheckChangeOnUpload(input_api, output_api):
   return CommonChecks(input_api, output_api)
 
 
 def CheckChangeOnCommit(input_api, output_api):
   return CommonChecks(input_api, output_api)
 
 
 def GetPreferredTryMasters(project, change):
   return {
-    'tryserver.chromium': {
+    'tryserver.chromium.linux': {
       'linux_nacl_sdk': set(['defaulttests']),
       'linux_nacl_sdk_build': set(['defaulttests']),
+    },
+    'tryserver.chromium.win': {
       'win_nacl_sdk': set(['defaulttests']),
       'win_nacl_sdk_build': set(['defaulttests']),
+    },
+    'tryserver.chromium.mac': {
       'mac_nacl_sdk': set(['defaulttests']),
       'mac_nacl_sdk_build': set(['defaulttests']),
     }
   }